This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present invention which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Some microwave radio frequency (RF) devices have very low tolerances to ESD shocks. In fact, an ESD pulse having a magnitude of no more than 100 or 200 volts can damage sensitive components of some RF devices. This is unfortunate, because some RF applications require resistance to much greater ESD discharges, even up to and above 15 kV. In addition, many RF devices are very sensitive to shunt capacitance because of their operating frequencies. This sensitivity to shunt capacitance presents an added requirement for an ESD protection circuit to have very low capacitance, perhaps less than one picofarad.
A typical system approach is to guide ESD energy away from a signal path to a return (ground) path as quickly as possible, in order to shield any ESD-sensitive component connected to the signal path from the ESD signal. In particular, one known approach is to employ shunt devices to ground. These shunt devices may include inductors, polymer devices, and spark gap devices. The shunt inductor approach can be utilized as an RF choke, where the RF impedance is large enough to not to influence the RF performance.
The fundamental transient equation for an inductor is set forth below:
                              V          ⁡                      (            t            )                          =                  L          *                                    ⅆ                              I                ⁡                                  (                  t                  )                                                                    ⅆ              t                                                          Equation        ⁢                                  ⁢        1            where V(t) is the voltage across an inductor at time t and where L is the inductance of the inductor. According to this equation, an inductor is capable of responding to an instantaneous voltage change, but is not capable of responding to an instantaneous current change. Because an inductor does not have a trigger voltage (as do polymer and spark gap ESD devices), the inductor begins to return energy to ground immediately, subject to the maximum current of its windings. For this reason, low capacitance polymer and spark gap ESD devices are typically placed in parallel with an inductor to assist in increasing the current capacity of the overall ESD protection circuit. Such typical ESD protection circuits, however, may not provide robust enough protection for RF components in hostile ESD environments.